Now resolved, converted the ascii string to hex string as advised in another thread, so in essence:
option 191 ascii pool1=10.8.48.116,10.8.48.117
converts to:
option 191 hex 61:73:63:69:69:20:70:6f:6f:6c:31:3d:31:30:2e:38:2e:34:38:2e:31:31:36:2c:31:30:2e:38:2e:34:38:2e:31:31:37